292 FRANZ SCHRADER 
process which again duplicates that found at that stage in those 
groups, only one functional (female-producing) spermatozoon 
being evolved. 
There is no need to enter into details in the well-known cases of 
Phylloxera and Aphids (v. Baehr, 09; Morgan, ’09). Briefly, 
it might be said that diploid parthenogenesis occurs and only one 
polar body is given off. In the female eggs this single division 
is equational, but in the male eggs certain sex chromosomes are 
cast out entire, and reduction therefore occurs as far as they are 
concerned. 
E. Kriiger (13) has described a case of semiparthenogenesis 
in the nematode, Rhabditis aberrans, in which the egg gives off 
a single polar body and the division is equational. The spermato- 
zoon enters the egg and starts development, but it does not con- 
jugate with the egg nucleus and finally degenerates. Practic- 
ally all worms found are hermaphrodites, and males are extremely 
rare. 
In the trematode, Diplodiscus temporatus (Cary, ’09), par- 
thenogenetic eggs arising in the sporocyst give off only one polar 
body and no reduction seems to occur, as might be expected. 
This, of course, is a case of obligatory parthenogenesis. 
Finally, mention may be made of artificial parthenogenesis. 
The starfish egg can be stimulated to develop before the polar 
bodies have been given off. Maturation will proceed, but ac- 
cording to Buchner (’15) the second polar body rejoins the nu- 
cleus, and segmentation stages show the diploid number of 
chromosomes. Unfortunately, no larvae have been reared to 
the stage where sex could be determined. Annelid eggs, which 
are also capable of being stimulated before polar body formation, 
have likewise not been reared to the necessary stage. 
The sea-urchin egg cannot thus be stimulated unless the polar 
bodies have already been given off. Development therefore 
proceeds with the haploid number, and Delage has succeeded in 
raising two such embryos to a stage where he could see that one 
was certainly a male and the other probably so. Such numbers 
are of course too small to admit of any definite conclusion. 
